Solar cell with a back-surface field method of production

ABSTRACT

For the simple production of a back surface field it is proposed that a boron-containing diffusion source layer (2) be applied to the rear (RS) of a silicon wafer (1) and boron be driven into the wafer to a depth of about 1 to 5 μm at 900 to 1200° C. This is done in an oxygen-containing atmosphere so that an oxide layer (4) is formed on open silicon surfaces, obviating the need to mask the regions not to be doped. After the removal of the oxide and source layer, phosphorus diffusion takes place and the back contact (3) is produced. It contains aluminum and, during the burn-in process, provides good ohmic contact.

A solar cell with a back-surface field and a method of its production.

BACKGROUND OF THE INVENTION

When attempting to reduce the thickness of silicon solar cells, adecreasing efficiency of the solar cell is observed. This is due, on theone hand, to the no longer complete absorption of sunlight when there isa thinner absorption length. On the other hand, charge carriers areincreasingly generated in proximity to the back, with minority chargecarriers being able to reach the back electrode as a result ofdiffusion, thereby reducing the current generated by the majority chargecarriers.

A highly doped layer on the back makes it possible to generate a fieldthat counteracts the diffusion of minority charge carriers, a so-calledback-surface field. In the case of a solar cell structure having ap-doped solar cell body and an n+ doped emitter on the solar cell'slight-incidence or front side, a p+ doping is necessary on the back forthis purpose. Aluminum, which is applied as a thin layer for example byvapor deposition on the back and which can be driven in or alloyed bymeans of an annealing step, is very often suggested in order to generatethis p+ doping. It is also possible to generate the p+ doping byapplying back contacts that contain aluminum and by correspondinglydriving in the aluminum. It is further possible to diffuse aluminum froma solid diffusion source into the solar cell substrate. Thisnevertheless suffers from the drawback that the solar cell substrate isdoped with aluminum on both sides, thus generating a p+pp+ structure.

Boron is also suitable for p-doping generation. A correspondingback-surface field can be generated by gaseous diffusion of acorrespondingly volatile or gaseous boron compound, by applying asilicon layer that contains boron on the back or by applying a liquidsolution that contains a dopant. Due to the high volatility of the boroncompounds, however, an all-over diffusion which has to be prevented bymasking those solar cell regions that are not to be doped is constantlyobserved at the temperatures necessary for driving in the doping.

The p+ doping--which is simple to produce according to the process--withaluminum suffers from the disadvantage of an increased susceptibility tocorrosion. Over time, layer regions that contain aluminum may decomposeand peel off, which may result in damage to the back contacts and causea reduction in solar cell performance.

A method of producing a silicon solar cell is known from IEEEPHOTOVOLTAIC SOLAR ENERGY CONFERENCE, Oct. 12-16, 1992, Montreux,Switzerland, pages 164-167; in this method, a boron-doped oxide layer isapplied on the back of a silicon wafer and then the boron is diffusedinto the silicon at a temperature of 940° C. An emitter layer issubsequently generated by means of phosphorus diffusion before contactsare applied by etching.

A method of producing a silicon solar cell which is particularlydirected at the production of doped regions is known from PCT referenceWO91/19323. An oxide-forming mask layer containing a dopant is appliedto part of the surface of a semiconductor substrate, and the substrateis then heated to a temperature sufficient for the diffusion of part ofthe dopant from the mask layer into the semiconductor layer, with thebare semiconductor substrate surface also being autodoped. Thesemiconductor substrate's autodoped regions are etched off, while themask layer represents a protective layer for the doped regions beneaththe mask layer.

SUMMARY OF THE INVENTION

The problem facing the present invention is to indicate a method ofproducing a back-surface field in a silicon solar cell; such a methodcan be integrated without considerable outlay into a conventional solarcell production process that is simple and reliable to implement andwhich results in a solar cell that is stable over the long term andexhibits reduced susceptibility to corrosion. The method is intended tomake it possible to economize on silicon material by means of thinnersolar cells while nevertheless achieving higher solar cell efficiency.

In general terms the present invention is a method of generating a solarcell with a back-surface field. In a step (a) a diffusion source layerthat contains boron as a dopant is applied on the back of a p-dopedsilicon wafer. In a step (b) the wafer is treated in an atmosphere thatcontains oxygen at a temperature of 900° C. to 1200° C. to generate anoxide layer and to drive in the dopant. In a step (c) the diffusionsource layer and the oxide layer are removed. There is in a step (d) anall-over diffusion of phosphorus to generate an n+ doped emitter layer.In a step (e) the n+ doped layer is separated at the edge of the wafer.A contact that contains aluminum is applied in a step (f) and in a step(g) is burned in. In a step (h) a front side contact is produced.

Advantageous developments of the present invention are as follows.

A boron doping resist layer is applied in a step (a) as a diffusionsource layer.

The wafer is exposed to a temperature of 1000° C. to 1100° C. in step(b).

The diffusion source layer and the oxide layer are removed by etchingusing HF solution in step (c).

The wafers are densely stacked above or next to one another in step (e)and the separation of the p+ doped layer takes place by etching off theouter edges of the wafers.

A back contact is applied by imprinting a silver screen printing paste.

A back contact which contains 1 to 3 percent by weight aluminum isapplied in step (f).

The boron is driven to a depth of 1 μm to 5 μm in step (b).

The present invention is also a solar cell having a p-doped solar cellbody, a layer region p+ doped with boron with a depth of 1 μm to 5 μm onthe back, an n+ doped layer region at least on the front side, a frontside contact, a burned-in silver back contact that contains aluminum,and an aluminum-doped connection region in the region of the backcontact.

In a further development of the solar cell the back contact is notholohedrally applied. The boron doping is overcompensated by a flattern+ doping between those regions of the back covered by the back contact.

In another development of the solar cell, the solar cell has ananti-reflective layer on the front and an oxide layer as a passivatinglayer on the back.

The invention's basic idea is to generate the back-surface field's p+doping by driving boron out of a boron-containing diffusion sourcelayer. The undesirable doping of the wafer's edges and front side isprevented in that the driving out operation is performed at hightemperatures of 900 to 1200° C. in an atmosphere that contains oxygen.Under these conditions, an oxide layer is immediately formed at thewafer's edges and front side; this layer serves the function of masking,thereby preventing undesirable doping at these sites. After driving in,both the oxide and the diffusion source layer can be removed as a resultof a simple etching step.

The back-surface field (BSF) is therefore generated before thesemiconductor junction is produced, i.e. before the diffusion ofphosphorus on the front side of the solar cell. The selected hightemperatures ensure that the boron doping is driven in deeply. Thislatter is then likewise stable with respect to all of the solar cell'sfollowing production steps which are carried out at much lowertemperatures.

A further advantage of the invention is obtained when producing thejunction by means of diffusion of phosphorus, which may take place allover, i.e. on both sides and at the edges of the wafer. With regard tophosphorus diffusion, it is therefore necessary neither to mask nor tocover layer regions in order to rule out undesirable doping in specificregions. The deeply driven in BSF doping is only superficiallyovercompensated by the phosphorus.

By using the back contact's aluminum-containing material, it is in turnpossible to contact through to the p+ layer when burning in the backcontact and to recompensate the n+ layer in the back contact region. Theback contact can be applied in a structured or holohedral manner.

The diffusion source layer is one that contains boron and out of whichthe boron is thermally driven. The diffusion source layer is preferablyapplied by means of a boron doping resist. In addition to boron or itscompounds, this resist contains powdery SiO₂ in a suspension. Thisdoping resist is normally used for generating high dopings in powersemiconductors. It may be applied in liquid form and may for example bespun on.

The back of the solar cell is preferably hydrophobic and henceoxide-free. The doping resist is applied to this surface in as thin alayer as possible and dried, thus preventing the diffusion source layerfrom forming cracks or even flaking off during the driving-in process.The back is homogeneously doped with a homogeneous and undamageddiffusion source layer.

The boron from the diffusion source layer is driven into the solar cellat 900 to 1200° C., preferably at 1000 to 1100° C. This range is belowthe 1280° C. suggested by the doping resist manufacturer for applicationpurposes. The driving-in temperature is nevertheless higher than waspreviously the case with solar cells.

As regards the application of boron doping resist, its manufacturer hadpreviously suggested coating one side of the components or wafers withthe resist and placing them on top of each other in a stack such thatsurfaces respectively to be doped or not to be doped end up on top ofone another. In this way, doping of the respectively opposite surface isintended to be prevented without necessitating masking. This proposedmethod does, however, have the drawback that the components or wafersagglomerate at the high requisite drive-in temperatures and must then bemechanically or chemically separated from one another.

In the method according to the invention, those regions that are not tobe doped need neither be masked nor covered by stacking. When the dopingis driven in, the solar cells (wafers) are spaced apart so that noagglomeration can occur.

When the doping is driven in, the atmosphere has to contain oxygen. Theprocess is preferably carried out in a pure oxygen atmosphere. In orderthat the oxide layer can be immediately formed, the solar cells aredirectly introduced into a furnace preheated to the drive-intemperature. After the oxide's rapid formation, the oxygen atmospherecan be replaced by other gases such as nitrogen.

Another advantage of the method according to the invention is obtainedfrom an oxide layer's high affinity vis-a-vis boron which is greaterthan that of silicon with respect to boron. This causes volatile boronescaping into the atmosphere to be optionally absorbed by the oxidelayer while the doping is being driven and allows the boron to penetrateonly slightly the surface regions to be excluded from boron doping.

After cooling down, both the diffusion source layer and oxide layer areremoved, for instance by a HF dip.

The semiconductor junction necessary for the solar cell is generated byphosphorus diffusion. This may take place by means of all-overdiffusion, whereby in addition to the n+ doped emitter layer on thefront side, a flat n+ doped layer region is produced on the back. Sincea much lower temperature than when the boron is driven in is set duringphosphorus diffusion at about 800 to 900° C., the p+ doping, which ismuch deeper at 1 to 5 μm, is retained beneath the approx. 0.2 μm deep n+doping on the back of the solar cell.

To obtain a functional semiconductor component, the p+ doping has to beseparated at the edge of the solar cell.

This avoids short circuits and resultant power losses of the solar cell.For the purpose of separating, the solar cells can be stacked on top ofone another and are exposed to an etching plasma for a short time.

To obtain a functional back contact, the p+ layer has to be contactedthrough the n+ layer on the back. This is achieved using a back contactwhich contains approx. 1 to 3 percent by weight aluminum. When burningin the back contact, the aluminum penetrates the back of the solar celland generates a p+ doping there which overcompensates the n+ dopingbeneath the back contact. A low-resistance connection region whichensures good current drainage during the solar cell operating mode istherefore produced beneath the back contact.

The current-draining contact that is still absent for a functional solarcell and which is located on the front side (front-side contact) can beproduced in a known manner before, during or at the same time as theback contact or after burning in the back contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIGS. 1 to 4 show various procedural stages based on schematic crosssections through a solar cell.

FIGS. 5 and 6 show solar-cell doping profiles generated according to theinvention and

FIG. 7 shows a finished solar cell in schematic cross section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 a <100> orientation p-doped Cz silicon wafer is for examplechosen for the solar cell. In this wafer, texturing which improves thegeometry of incidence of light in order to prevent reflection (not shownin FIG. 1) can be produced on the surface by means of a brief, alkalinecrystal-oriented etching.

A thin doping resist layer 2 (e.g. Siodop®, Merck company) is now spunon the back RS and dried.

FIG. 2 the wafer prepared in this manner is now set into a tray orhurdle and placed in a furnace preheated to 1000 to 1100° C. A pureoxygen atmosphere is set inside the furnace so that an oxide layer 4 isformed directly on all those surfaces of the wafer 1 that are notcovered by the boron doping resist layer 2. The boron is simultaneouslydriven out of the doping resist layer 2 and diffuses into the back RS ofthe wafer 1. A p+ doped area 5 with a depth of approx. 1 to 5 μm isformed.

An oxide layer 4 and doping resist layer 2 are removed from the waferusing a HF dip.

FIG. 3 a flat n+ doped surface region 7 is now produced all over as aresult of phosphorus diffusion (see arrow 6). The conditions are setsuch that the n+ doped region 7 reaches a depth of approx. 1 μm,preferably 0.2 μm.

FIG. 4 after separating the n+ region 7 at the edge of the disk as aresult of etching off (for example in a plasma), a back contact 3 isapplied. This takes place for example by means of screen printing usinga paste in which as well as binding agents and oxidic additions,conductive silver particles and 1 to 3% by weight aluminum are present.After imprinting, the back contact 3 is burnt in at approx. 700 to 800°C. The dopant aluminum additionally present in the paste is driven intothe solar cell back where in the connection region 8, it ensures a p+doping by overcompensation of the n+ doping, thereby ensuring good ohmiccontact between the p+ area 5 and the back contact 3.

FIG. 5 schematically depicts the generated doping profile in the solarcell before the back contact is burned in. The doping concentration isapplied against the disk thickness between back RS and front side VS.The region 1 represents the wafer's low even original p doping. Theregion 5 characterizes the p+ doping which is driven about 5 μm into thewafer's back RS. The n+ doping 7 generated by phosphorus diffusion andhaving a low penetration depth of about 0.2 μm forms the emitter on thefront side and likewise generates an n+ doped region on the back byovercompensation of the p+ doping.

FIG. 6 depicts the doping profile after the back contact has been burnedin, whereby the back contact is located in the sectional region of thedepicted cross-sectional plane. The n+ doping of the back isovercompensated by the aluminum in the connection region, thus producinga continuously p+ doped region 8 beneath the back contact. A good ohmiccontact between the imprinted and burned-in back contact and the solarcell is therefore ensured.

FIG. 7 shows a schematic cross section of a solar cell that is completedby means of procedural steps known per se. This cell comprises a frontside contact 9 and optionally an antireflective layer 10 on the frontside which can be formed from an oxide or silicon nitride, as well as aback passivating layer 11, for example a passivating oxide. These twolayers can optionally be produced before applying the front and/or backcontact. Due to the high surface doping, an oxide grows particularlyrapidly, with the result that moderate temperatures and short processtimes are themselves sufficient for passivation.

The invention is not limited to the particular details of the method andapparatus depicted and other modifications and applications arecontemplated. Certain other changes may be made in the above describedmethod and apparatus without departing from the true spirit and scope ofthe invention herein involved. It is intended, therefore, that thesubject matter in the above depiction shall be interpreted asillustrative and not in a limiting sense.

We claim:
 1. A method of generating a solar cell with a back-surfacefield comprising the steps of:a) applying a diffusion source layer thatcontains boron as a dopant on a back side of a p-doped silicon wafer; b)treating said wafer in an atmosphere that contains oxygen at atemperature of 900 to 1200° C. to generate an oxide layer; c) removingsaid diffusion source layer and said oxide layer; d) diffusingphosphorus on all sides of the wafer to generate an n+ doped emitterlayer; e) separating said n+ doped layer at an edge of said wafer; f)applying a back contact that contains aluminum on the back side; g)burning in said back contact; and h) producing a front side contact on afront side of said wafer.
 2. The method according to claim 1, wherein aboron doping resist layer is applied in step a) as the diffusion sourcelayer.
 3. The method according to claim 1, wherein said wafer is exposedto a temperature of 1000 to 1100° C. in step b).
 4. The method accordingto claim 1, wherein the diffusion source layer and the oxide layer areremoved by etching using HF solution in step c).
 5. The method accordingto claim 1, wherein a plurality of wafers are provided and wherein saidwafers are densely stacked above or next to one another in step e) andwherein the separation of said n+ doped layer takes place by etching offthe outer edges of said wafers.
 6. The method according to claim 1,wherein the back contact is applied by imprinting a silver screenprinting paste.
 7. The method according to claim 1, wherein a backcontact which contains 1 to 3 percent by weight aluminum is applied instep f).
 8. The method according to claim 1, wherein the boron is drivento a dept of 1 to 5 μm in step b).